In a packet, frame or cell switched data transmission network, digital data is transmitted in data transmission packets, cells or frames. Below in this document, a data transmission packet, cell and frame are called protocol units. Protocol units used in a data transmission service can be either standard-sized or with an alternating size. A data transmission network typically causes random-type fluctuation in the transmission delay, because protocol units must queue in queue buffers of network elements provided in the data transmission route for durations determined by the momentary load situation. Said network elements can be for example IP routers (Internet Protocol), ATM switches (Asynchronous Transfer Mode), MPLS switches (MultiProtocol Label Switching) or Ethernet switches. The effect of the fluctuation in the transmission delay can be eliminated, or it can be alleviated by using a jitter buffer at the reception end, in which jitter buffer the digital data contained by the received protocol units is stored, and from which said digital data is read by using a clock signal where the fluctuation of the frequency is sufficiently slight, and the timewise average of the frequency of said clock signal corresponds to the average arrival rate of said digital data. Thus the jitter buffer absorbs the fluctuation of the transmission delay either completely or partly, so that the quantity of the digital data contained by the jitter buffer, i.e. the degree of filling of the jitter buffer, varies. The average degree of filling of the jitter buffer must be sufficiently high, in order to ensure that the density of occurrence of situations where the digital data read-out sequence begins already before said digital data has arrived in the jitter buffer. On the other hand, the average degree of filling of a jitter buffer should not be unnecessarily high, in order to avoid an excessively long digital data turn-around delay from being created in the jitter buffer. In connection with the manufacturing and/or implementation of a network element, the maximum and minimum values of the transmission delays experienced by protocol units to be received in the future are generally not known. In addition, the distribution of transmission delays caused by the data network can alter during the service time of a network element, for example due to rerouting arrangements. Consequently, a network element is often provided with adjusting devices, the task of which is to adjust the degree of filling of a jitter buffer within the scope of the above described boundary conditions.
The publication US2005/0058146 A1 introduces a solution where the degree of filling of a jitter buffer is increased as a response to a situation where in the jitter buffer there is not found a data packet, the digital data contained by which would be in turn to be read out. The process of increasing the degree of filling of a jitter buffer is carried out by feeding in the jitter buffer filler packets that can be for instance data packets containing digital data that has been read out at an earlier stage. The suggested solution is based on the fact that the data packet that is in turn to be read out is not found in the jitter buffer, when the transmission delay experienced by said data packet is longer than can be compensated by the degree of filling of the jitter buffer. Now the missing of said data packet indicates a need to increase the degree of filling of the jitter buffer. Likewise, when a data packet to be transmitted has been dropped, for example because of a protocol error or a traffic jam in a network element located in the data transmission route, there arises a situation where in the jitter buffer, there is not found the data packet, the digital data contained by which would be in turn to be read out. Thus also the dropping of a data packet dropping results in increasing the degree of filling of the jitter buffer.
A dropped data packet never reaches its destination, and therefore the transmission delay experienced by it is theoretically infinite. However, the dropping of a data packet does not increase the fluctuation of the transmission delay of those data packets that were transmitted successfully. Often the case can be even the opposite, because the dropping of a data packet reduces the traffic jam experienced by other data packets. The purpose in adjusting the degree of filling of a jitter buffer is, however, to facilitate an adequate, high-quality processing for those data packets that were transmitted successfully. In some cases, increases in the degree of filling of a jitter buffer caused by the dropping of data packets can result in an unnecessarily long turn-around delay of digital data in said jitter buffer.